1. Field of the Invention
This invention generally relates to an electrophotographic copying apparatus, and more particularly to the cleaning of the surfaces of a photosensitive medium and image transfer device in such copying apparatus.
2. Description of the Prior Art
Electrophotographic copying apparatuses have recently been widely in use because of their capability of producing purely black-and-white copies at very high speeds and in a very simple manner. The electrophotography originated from the xerography invented by C. F. Carlson in 1938, and since then various versions of it have been developed and put into practice. These various electrophotographic processes may generally be grouped into two types, one of which is the system whereby an electrostatic latent image is formed on a photosensitive sheet of paper or the like, whereafter the formed image is developed with coloring toner and fixed directly on the sheet to provide a final copy. The other type is known as the image transfer system whereby an electrostatic latent image is formed on a photosensitive medium and then developed with coloring toner, whereafter such developed image is transferred to a separate transfer medium such as paper or the like and is fixed thereon to complete a final copy. The residual developer on the surface of the photosensitive medium is removed by cleaning the same for reuse. The former system has required the paper or the like to be coated with a photosensitive substance such as zinc oxide in order to impart the required photosensitivity. This has necessarily led to a higher cost and greater weight of the photosensitive sheet. The latter system is quite free of such disadvantages. In the transfer type electrophotographic copying apparatus, various types of image transfer means have heretofore been proposed and put into practice. In general, two types of transfer systems are known. First, there is the corona transfer system whereby an electrostatic latent image formed on a photosensitive medium is developed, an image transfer medium is afterwards superposed on the developed image, and then a corona discharge opposite in polarity to the charge in the toner is applied to the transfer medium from behind to thereby accomplish image transfer. Second, there is the roller transfer system whereby an electrically conductive roller with a voltage applied thereto is urged into contact with the back side of the transfer medium superposed on the developed image, to thereby achieve image transfer.
The first-named corona transfer system is meritorious in that (1) the photosensitive medium is free of any flaw or injury because the transfer means is not brought into physical contact with the photosensitive medium, and (2) the transfer medium is electrostatically urged against the photosensitive medium without any image transfer means being brought into contact with the back side of the transfer medium, thus avoiding any stain produced on the back side of the transfer medium due to the coloring toner. On the other hand, however, this system suffers from these demerits: (1) the transfer pressure of the transfer medium relative to the developed image on the photosensitive medium is so weak as to allow any slightest wrinkle or the like in the transfer medium to cause an air gap between the transfer medium and the developed image, thus resulting in an irregular image transfer effect, and this also imposes limitations on the type of the transfer medium and humidity because thin paper or high humidity would only result in poor image transfer effect: (2) the corona discharge itself is readily affected by temperature or humidity: and (3) the corona discharge electrode tends to be stained with toner and this causes an irregular discharge effect which in turn would result in an irregular transfer effect.
The second-named roller transfer system enjoys these advantages: (1) the transfer medium is sufficiently urged into contact with the photosensitive medium by the transfer roller so that the toner and the transfer medium intimately contact each other, thereby providing a good transfer image with the aid of the electric field applied to the transfer roller: (2) more or less wrinkles formed in the transfer medium under higher humidities could lead to no irregular image transfer because the transfer roller ensures a sufficiently intimate contact between the transfer medium and the developed image: and (3) ambient conditions such as temperature or humidity changes, and toner or like contaminant can in no way affect the transfer medium, thus avoiding any irregularity of the transfer effect. However, these advantages are accompanied by the following disadvantages: (1) if the transfer roller was stained with toner or like contaminant, it would stain the back side of the transfer sheet during the transfer process, thus providing an ugly final copy: and (2) if the transfer roller had a high degree of hardness, it would injure the photosensitive medium.